Ecological Applications, 12(5), 2002, pp. 1247–1260
q 2002 by the Ecological Society of America
ESA Report
MEETING ECOLOGICAL AND SOCIETAL NEEDS FOR FRESHWATER
JILL S. BARON,1,11 N. LEROY POFF,2 PAUL L. ANGERMEIER,3 CLIFFORD N. DAHM,4 PETER H. GLEICK,5
NELSON G. HAIRSTON, JR.,6 ROBERT B. JACKSON,7 CAROL A. JOHNSTON,8
BRIAN D. RICHTER,9 AND ALAN D. STEINMAN10,12
1U.S.
Geological Survey, Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado 80523 USA
2Department of Biology, Colorado State University, Fort Collins, Colorado 80523 USA
3U.S. Geological Survey, Virginia Cooperative Fish and Wildlife Research Unit,
Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 USA
4Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131 USA
5Pacific Institute for Studies in Development, Environment, and Security, 654 13th Street, Oakland, California USA 94612 USA
6Section of Ecology and Systematics, Cornell University, Ithaca, New York 14853 USA
7Department of Biology and Nicholas School of the Environment, Duke University,
Durham, North Carolina 27708 USA
8Division of Environmental Biology, National Science Foundation, 4201 Wilson Boulevard, Arlington, Virginia 22230 USA
9The Nature Conservancy, 490 Westfield Road, Charlottesville, Virginia 22901 USA
10South Florida Water Management District, P.O. Box 24680, West Palm Beach, Florida 33416 USA
Abstract. Human society has used freshwater from rivers, lakes, groundwater, and
wetlands for many different urban, agricultural, and industrial activities, but in doing so
has overlooked its value in supporting ecosystems. Freshwater is vital to human life and
societal well-being, and thus its utilization for consumption, irrigation, and transport has
long taken precedence over other commodities and services provided by freshwater ecosystems. However, there is growing recognition that functionally intact and biologically
complex aquatic ecosystems provide many economically valuable services and long-term
benefits to society. The short-term benefits include ecosystem goods and services, such as
food supply, flood control, purification of human and industrial wastes, and habitat for plant
and animal life—and these are costly, if not impossible, to replace. Long-term benefits
include the sustained provision of those goods and services, as well as the adaptive capacity
of aquatic ecosystems to respond to future environmental alterations, such as climate change.
Thus, maintenance of the processes and properties that support freshwater ecosystem integrity should be included in debates over sustainable water resource allocation.
The purpose of this report is to explain how the integrity of freshwater ecosystems
depends upon adequate quantity, quality, timing, and temporal variability of water flow.
Defining these requirements in a comprehensive but general manner provides a better
foundation for their inclusion in current and future debates about allocation of water resources. In this way the needs of freshwater ecosystems can be legitimately recognized and
addressed. We also recommend ways in which freshwater ecosystems can be protected,
maintained, and restored.
Freshwater ecosystem structure and function are tightly linked to the watershed or
catchment of which they are a part. Because riverine networks, lakes, wetlands, and their
connecting groundwaters, are literally the ‘‘sinks’’ into which landscapes drain, they are
greatly influenced by terrestrial processes, including many human uses or modifications of
land and water. Freshwater ecosystems, whether lakes, wetlands, or rivers, have specific
requirements in terms of quantity, quality, and seasonality of their water supplies. Sustainability normally requires these systems to fluctuate within a natural range of variation.
Flow regime, sediment and organic matter inputs, thermal and light characteristics, chemical
and nutrient characteristics, and biotic assemblages are fundamental defining attributes of
freshwater ecosystems. These attributes impart relatively unique characteristics of productivity and biodiversity to each ecosystem. The natural range of variation in each of these
attributes is critical to maintaining the integrity and dynamic potential of aquatic ecosysManuscript received 17 August 2001; revised 9 January 2002; accepted 11 January 2002.
Reprints of this 14-page ESA Report are available for $2.25 each. Prepayment is required. Order reprints from the Ecological
Society of America. Attention: Reprint Department, 1707 H Street, N.W., Suite 400, Washington, DC 20006.
11 E-mail: [email protected]
12 Present address: Annis Water Resources Institute, Lake Michigan Center, 704 West Shoreline Drive, Muskegon, Michigan
49441 USA.
1247
ESA REPORT
1248
Ecological Applications
Vol. 12, No. 5
tems; therefore, management should allow for dynamic change. Piecemeal approaches cannot solve the problems confronting freshwater ecosystems.
Scientific definitions of the requirements to protect and maintain aquatic ecosystems
are necessary but insufficient for establishing the appropriate distribution between societal
and ecosystem water needs. For scientific knowledge to be implemented science must be
connected to a political agenda for sustainable development. We offer these recommendations as a beginning to redress how water is viewed and managed in the United States:
(1) Frame national and regional water management policies to explicitly incorporate freshwater ecosystem needs, particularly those related to naturally variable flow regimes and to
the linking of water quality with water quantity; (2) Define water resources to include
watersheds, so that freshwaters are viewed within a landscape, or systems context; (3)
Increase communication and education across disciplines, especially among engineers, hydrologists, economists, and ecologists to facilitate an integrated view of freshwater resources; (4) Increase restoration efforts, using well-grounded ecological principles as guidelines; (5) Maintain and protect the remaining freshwater ecosystems that have high integrity;
and (6) Recognize the dependence of human society on naturally functioning ecosystems.
Key words: ecological education; ecological integrity; ecosystem protection; ecosystem services,
freshwater; freshwater ecosystems; lakes; restoration; rivers; waterflow: quantity, quality, timing,
seasonality; water management policy; watersheds; wetlands.
INTRODUCTION
Human society has used freshwater from rivers,
lakes, groundwater, and wetlands for many different
urban, agricultural, and industrial activities, but in doing so has overlooked its value in supporting ecosystems. Aquatic ecosystems are being severely altered or
destroyed at a greater rate than at any other time in
human history, and far faster than they are being restored (NRC 1992). Freshwater is vital to human life
and societal well-being, thus its utilization for consumption, irrigation, and transport has long taken precedence over other commodities and services provided
by freshwater ecosystems. However, there is growing
recognition that functionally intact and biologically
complex aquatic ecosystems provide many economically valuable services and long-term benefits to society. Daily (1997:3) defines an ecosystem service as
‘‘the conditions and processes through which natural
ecosystems, and the species that make them up, sustain
and fulfill human life.’’ Short-term benefits include
ecosystem goods and services, such as drinking water,
food supply, flood control, purification of human and
industrial wastes, and habitat for plant and animal life
—all of which are costly, if not impossible, to replace
(Wilson and Carpenter 1999, Daily 1997). Long-term
benefits include the sustained provision of those goods
and services, as well as the adaptive capacity of ecosystems to respond to future environmental alterations,
such as climate change. Thus, maintenance of the processes and properties that support freshwater ecosystem
integrity should be recognized as legitimate goals for
freshwaters, worthy of consideration in debates over
sustainable water resource allocation (NRC 1992, Karr
1991, Naiman et al., in press). Human society is served
in the long term by ecosystem sustainability. We must
develop a coherent policy that more equitably allocates
water resources between natural ecosystem function
and societal needs.
Current water management policies are clearly unable to meet this goal. United States laws and regulations for water are implemented in a management
context that focuses primarily on the lowest acceptable
water quality, minimal flows, and single-species protection. Literally dozens of different government entities in the United States have a say in what goes into
water or how water is used and redistributed, and the
goals of one agency are often at cross-purposes with
those of others (van der Leeden et al. 1990). A fundamental change in water management policies is needed, one that embraces a much broader view of the dynamic nature of freshwater resources and the short- and
long-term benefits they provide.
Our educational practices are equally inadequate to
the challenge of sustainable water resource management. Hydrologists, engineers, and water managers, the
people who design and manage our nation’s water resource systems, are rarely taught about management
consequences to ecosystems, nor are ecologists trained
to think about the critical role of water in human society. Economists, developers, and politicians seldom
project far enough into the future to fully account for
the potential ecological costs of short-term plans. Few
Americans are aware of the infrastructure that brings
them pure tap water or carries their wastes away, and
fewer still understand the ecological trade-offs that
were made to allow these conveniences. How can society extract the water resources it needs while not
diminishing the important natural complexity and
adaptive capacity of freshwater ecosystems? The requirements of freshwater ecosystems are often at odds
with human activity, although this need not always be
the case. Our present state of ecological understanding
of how freshwater ecosystems function allows us to
elaborate the requirements of freshwater ecosystems
regarding adequate quantity, quality, and timing of water flow. Communication of these requirements to a
October 2002
WHAT DO FRESHWATER ECOSYSTEMS NEED?
1249
TABLE 1. Changes in hydrologic flow, water quality, wetland area, and species viability in U.S. rivers, lakes, and wetlands
since Euro-American settlement.
Freshwater parameter
Pre-settlement
condition
Current conditions
Information source
Free-flowing river kilometers, 48 states
Number of dams .2 m high
Volume of water diverted from surface
waters, 1985
Total daily water use, 1995
Sediment inputs to reservoirs
River water quality, 1.1 3 106 km
surveyed†
Lake water quality, 6.8 3 106 ha
surveyed‡
Wetland area, 48 states
5.1 3 10 km
0
0
100 km
75 000
106 m3/d
Benke (1990)
CEQ (1995)
Solley et al. (1998)
unknown
not applicable
unimpaired
1.5 3 106 m3/d
1,200 3 106 m3/yr
402 000 km impaired
Solley et al. (1998)
Stallard (1998)
EPA (1998)
unimpaired
2.7 3 106 ha impaired
EPA (1998)
87 3 10 ha
35 3 10 ha
No. freshwater fish species
822 species
No. freshwater mussel species
305 species
No. crayfish species
330 species
No. amphibian species
242 species
202 species imperiled or
extinct
157 species imperiled or
extinct
111 species imperiled or
extinct
64 species imperiled or extinct
van der Leeden et al.
(1990)
Stein and Flack (1997)
6
6
6
Stein and Flack (1997)
Stein and Flack (1997)
Stein and Flack (1997)
† Only 19% (1 116 500 km) of total river kilometers in the United States were surveyed out of a total of 5 792 400 km.
‡ Only 40% (6.8 million ha) of total lake area (16.9 million ha) were surveyed.
broad community is a critical step for including freshwater ecosystem needs in future discussions of resource
allocations. The American public, when given information about management alternatives, supports ecologically based management approaches, particularly
toward freshwaters (CEQ 1996).
Several previous studies have addressed the overall
condition of freshwater resources, and have recognized
that: water movement through the biosphere is highly
altered by human activities (NRC 1992, Gleick 1993,
Jackson et al. 2001); water is intensively used by humans (Postel et al. 1996, Vitousek et al. 1997); poor
water quality is pervasive (Carpenter et al. 1998); and
freshwater biota are disproportionately imperiled by
societal activities (Dobson et al. 1997, Ricciardi and
Rasmussen 1999). These and other analyses indicate
that freshwater ecosystems are under stress and at risk
(Table 1). Clearly, new management approaches are
needed.
In this paper we describe the requirements of freshwater ecosystems for water of sufficient quality,
amount, timing, and temporal variability in order to
maintain the natural dynamics that produce ecosystem
services, and marketable and non-marketable goods.
We suggest steps to be taken toward restoration. The
paper concludes with recommendations for how to protect and maintain freshwater ecosystems.
THE REQUIREMENTS OF FUNCTIONALLY INTACT
FRESHWATER ECOSYSTEMS
Freshwater ecosystems differ greatly from each other
depending on type, location, and climate but share important features. Lakes, wetlands, rivers, and their connected groundwaters have a common need for water
within a certain range of quantity and quality. Because
freshwater ecosystems are dynamic, they additionally
require a range of natural variation or disturbance to
maintain viability, or resilience (Holling 1986, Pickett
et al. 1992). Both seasonal and interannual variability
in flow are needed to support biota and maintain natural
habitat dynamics that support production and persistence of species (Naiman 1992, Stanford et al. 1996,
Poff et al. 1997). The sizes of native plant and animal
populations and their age structures, the presence of
rare or highly specialized species, the interactions of
species with each other and their environments, and
many ecosystem processes are strongly influenced by
the temporally varying hydrologic regimes that characterize these ecosystems. Water quality, physical habitat conditions, and energy sources are shaped by periodic and episodic water-flow patterns. Natural freshwater ecosystems, therefore, have evolved to the
rhythms of hydrologic variability.
The structure and function of freshwater ecosystems
are tightly linked to the watershed, or catchment, of
which they are a part (Hynes 1970, Likens 1984). As
water flows on its way to the sea, it moves through
freshwater systems in three spatial dimensions: longitudinal (upstream–downstream), lateral (channel–
floodplain, or wetland–lake margin), and vertical (surface water–groundwater). These dimensions represent
functional linkages among ecosystem compartments
over time (Ward 1989). Bodies of freshwater are ultimately the recipients of materials generated from the
landscape, hence they are greatly influenced by terrestrial processes, including human modifications of land
(Moyle and Leidy 1992).
We identify five dynamic environmental drivers that
Ecological Applications
Vol. 12, No. 5
ESA REPORT
1250
FIG. 1.
Conceptual model of major driving forces that influence freshwater ecosystems.
regulate much of the structure and function of any
aquatic ecosystem, although their relative importance
varies among aquatic ecosystem types (Fig. 1). The
interaction of these drivers in space and time defines
the dynamic nature of freshwater ecosystems.
1) The flow regime defines the rates and pathways
by which precipitation enters and circulates within
river channels, lakes, wetlands, and connecting
groundwaters, and the residence time of water in
the ecosystem.
2) Sediment and organic matter inputs provide raw
material to create physical habitat structure, refugia, and nutrient storage and supply.
3) Thermal and light characteristics regulate organismal metabolism, activity level, and ecosystem
productivity.
4) Chemical and nutrient characteristics regulate pH,
productivity, and water quality.
5) The biotic assemblage influences ecosystem process rates and community structure.
In naturally functioning systems all five of these
drivers display natural annual periodicity according to
seasonal changes in climate and day length that define
a range of variation. Ecosystems and species have
evolved to accommodate annual cycles of these drivers,
and they have also developed strategies for surviving—
indeed requiring—periodic hydrologic extremes
caused by floods and droughts that may not occur every
year. All five drivers must be considered jointly when
evaluating freshwater ecosystem integrity; focusing on
one at a time will not yield a true picture of a functional
aquatic ecosystem.
Flow regime
The natural or historical flow regime can be described for streams, rivers, wetlands, and lakes (Richter
et al. 1996, Stanford et al. 1996, Poff et al. 1997).
Certain aspects of the flow regime, particularly for rivers, are critical for regulating biotic production and
diversity. These include base flow, annual or frequent
floods, rare and extreme flood events, seasonality of
flows, and annual variability (Box 1). Flow regime, or
hydroperiod, also has relevance for lakes and wetlands
by influencing circulation patterns, renewal rates, and
types and abundances of aquatic vascular plants (Vollenweider 1976, Van der Valk 1981). The flow regime
of a lake or wetland is a critical influence on biotic
productivity and is important to determining acceptable
nutrient loads from surrounding areas.
Many rivers now resemble elaborate plumbing
works, with the timing and amount of flow completely controlled, like water from a faucet, so as to
maximize the rivers’ benefits for humans. But while
modern engineering has been remarkably successful
at getting water to people and farms when and where
they need it, it has failed to protect the fundamental
October 2002
WHAT DO FRESHWATER ECOSYSTEMS NEED?
1251
BOX 1. Definition of Flow Conditions for Rivers and Streams
Base flow conditions characterize periods of low flow between storms. They define the quantity of water
in the channel and thus directly influence habitat suitability for aquatic organisms as well as the depth to
saturated soil for riparian species. The magnitude and duration of base flow varies greatly among different
rivers, reflecting differences in climate, geology, and land use.
Frequent (e.g., 2-yr return interval) floods reset the system by flushing fine materials from the streambed,
thus promoting higher production during base flow periods (e.g., Fisher 1983). High flows may also
facilitate dispersal of organisms both up- and downstream. In many cases moderately high flows inundate
adjacent floodplains and maintain riparian vegetation (Hupp and Osterkamp 1985, Sparks 1995).
Rare or extreme events, such as 100-yr floods, represent important reformative events for riverine
systems. They transport large amounts of sediment, often transferring it from the main channel to floodplains. Habitat diversity within the river is increased as channels are scoured and reformed, and successional
dynamics in riparian communities and floodplain wetlands are reset. Large flows can also remove species
that are poorly adapted to dynamic riverine environments, such as upland tree species, or nonnative species,
whose invasive success is often minimized by natural, high flows (Meffe 1984, Moyle and Light 1996).
The restriction of major floods by reservoirs plays an important role in the establishment and proliferation
of exotic species in many riverine systems (see Poff et al. 1997, Richter et al. 1997).
Seasonal timing of flows (especially high flows) is critical for maintaining many native species whose
reproductive strategies are tied to such flows. For example, some fish use high flows to initiate spawning
runs (Nesler et al. 1988). Along western rivers, cottonwood trees release seeds during peak snowmelt to
maximize the opportunity for seedling establishment (Scott et al. 1996). Changing the seasonal timing of
flows has severe negative consequences for aquatic and riparian communities (Wootton et al. 1996, Auble
et al. 1994).
Annual variation in flow is an important factor influencing riverine systems. For example, interannual
variation in runoff volume can maintain high species diversity (Toth 1995). Similarly, ecosystem productivity and trophic structure can vary in response to this interannual variation (e.g., Power et al. 1996).
ecological function of rivers and aquatic systems.
(Postel 1996:45)
Western rivers in the United States are prime examples of how flow manipulation can lead to multiple
damages to riverine and riparian processes and communities. Dampening of natural flow variability by
managing for only minimum flows has contributed to
the widespread loss of native fish species (Moyle and
Light 1996), and to the regeneration failure of native
cottonwoods that support a diverse riparian community
(Scott et al. 1996, 1999). Since the completion of the
Glen Canyon Dam (Arizona, USA) in 1963, measurable
flows of the Colorado River to its mouth at the Gulf
of California have occurred only six times. The wetland
area at the mouth of the river has decreased to 5800–
63 000 ha (depending on the year), compared with
250 000 ha of original wetlands (Glenn et al. 1996,
Postel et al. 1998). The lack of freshwater inflows has
contributed to the endangerment of a large number of
species in the Sea of Cortez, and the abundance of
bivalve mollusk populations has dropped 94% from
1950 values due to loss of benthic productivity (Postel
et al. 1998, Kowalewski et al. 2000).
Sediment and organic matter inputs
In riverine systems, sediment flux and organic matter
inputs are important components of habitat structure
and dynamics. Natural sediment regimes are those that
accompany natural flow variation. Natural organicmatter regimes include seasonal inputs from terrestrial
environments. Terrestrial organic matter inputs, especially in smaller rivers and streams, are particularly
important sources of energy and nutrition, while large,
coarse, woody material provides substrate and habitat
for organisms (Cummins 1974, Gregory et al. 1991).
In lakes and wetlands, all but the finest inflowing sediment is permanently stored, so that over time these
systems fill. The invertebrates, algae, bryophytes, vascular plants, and bacteria that populate the bottoms of
freshwater systems are responsible for much of the water purification, decomposition, and nutrient cycling
that occurs (Palmer et al. 2000). They are highly adapted to the specific sediment and organic matter conditions of their environment, as are many fish species,
and do not persist if changes in the type, size, or frequency of sediment inputs occur (Swanson et al. 1988,
Allan 1995).
ESA REPORT
1252
Humans have severely altered the natural rates of
sediment and organic matter supply to aquatic systems
in ways that both increase and decrease inputs (e.g.,
Waters 1995). Poor agricultural, logging, or housingdevelopment practices promote high rates of soil erosion. Sediment capture behind dams truncates normal
sediment supply to downstream reaches, erodes streambeds, degrades habitat, and prohibits flood events from
rejuvenating wetland and riparian areas (Patten et al.
2001). Stallard (1998) estimates that 1.2 3 109 m3 of
sediment accrues yearly into reservoirs of the United
States (Table 1). Siltation from agricultural, urban, construction, and unspecified non-point sources is the
cause of impairment for fully one quarter of all lakes
that do not meet their water quality standards (EPA
1998). Channel straightening, overgrazing of riparian
areas, and clearing of streamside vegetation reduce organic matter inputs, but also often increase erosion.
Thermal and light characteristics
Light and heat properties are influenced by climate
and topography, and by a waterbody’s chemical composition, suspended sediments, and primary productivity. Water temperature directly regulates oxygen concentrations, organism metabolism, and associated life
processes. The thermal regime greatly influences organismal fitness and, by extension, the distribution of
species in both space (e.g., along latitudinal and altitudinal gradients) and time (e.g., seasonal variation at
one location). In lakes particularly, the absorption of
solar energy and its dissipation as heat are critical to
development of thermal structure and water circulation
patterns (Wetzel 1983). These characteristics in turn
influence nutrient cycling, distribution of dissolved
gases and biota, and the behavioral adaptations of organisms.
Water temperature can change dramatically downstream of dams (Ward and Stanford 1979). Mean
monthly temperatures ranged between 28C in winter
and 188C in summer in the Green River, Utah, USA,
before completion of the Flaming Gorge Dam in 1962.
After dam closure the water temperatures below the
dam exhibited a much reduced annual range of mean
monthly temperatures between 48C and 98C (Vinson
2001). Species richness declined and 18 macroinvertebrate genera were extirpated; other species, notably
crustaceans, came to dominate the invertebrate fauna
(Vinson 2001). Aquatic insects have not recovered,
even after 20 yr of partial thermal restoration. Water
temperature also dropped in the Colorado River after
closure of the Glen Canyon Dam in 1963 and, along
with a dramatic increase in water clarity, this allowed
for development of a nonnative trout population and
an unusual food web more commonly found in Nearctic
regions (Shannon et al. 2001). Water clarity is now
routinely .7 m, whereas prior to dam closure the water
Ecological Applications
Vol. 12, No. 5
column was opaque with suspended sediments (Shannon et al. 2001).
Chemical/nutrient characteristics
Natural nutrient and chemical conditions are those
that reflect local climate, bedrock, soil, vegetation type,
and topography (EPA 2000). Natural waters can range
from clear, nutrient-poor rivers and lakes on crystalline
bedrock, to much more productive and chemically enriched freshwaters in catchments with productive soils
or limestone bedrock.
Cultural eutrophication occurs when additional nutrients from human activities substantially increase productivity beyond the original state (Carpenter et al.
1998). Eastern lakes demonstrate the consequences of
excess nutrients and toxic contaminants, as well as nonnative species introductions and overfishing. Lakes
Michigan, Huron, Erie, and Ontario suffer from all of
the above (Anderson et al. 2001). Onondaga Lake, New
York, USA, which was polluted with salt brine effluent
from a soda ash industry, responded with marked
changes in plankton and fish species composition, including invasive species (Auer et al. 1996, Hairston et
al. 1999). Nutrients contributed to 51% of the water
quality problems of U.S. lakes identified as impaired
in 1996 (EPA 1998). More than one half of agricultural
and urban streams sampled in the United States have
pesticide concentrations that exceed guidelines for the
protection of aquatic life (USGS 1999).
Biotic assemblage
The communities of species that reside in aquatic
ecosystems reflect the regional species pools (themselves a product of biogeographic history), and the species’ abilities to colonize and survive (see Tonn et al.
1990). The bounds of environmental variation established by flow, sediment, thermal, light, and nutrient
regimes, and the presence of and interactions with other
species in the system, dictate the suitability of the ecosystem for any particular species. Thus, both biotic and
abiotic controls and feedbacks operate to maintain a
diverse range of species that are involved in the critical
ecosystem processes of primary production, decomposition, and nutrient cycling. The capacity to adapt to
future environmental variation is, in part, underlain by
the apparent functional redundancy of ecological functions performed by species. High apparent redundancy
(i.e., biodiversity) affords a kind of insurance that ecological functions will continue during environmental
stress (Walker 1992, Mulder et al. 2001).
Human alteration of environmental conditions can
greatly change assemblage structure and ecosystem
function. Excessive stress or simplification of natural
complexity has the potential to push functionally intact
freshwater ecosystems beyond the bounds of resilience
or sustainability, threatening their ability to provide
important goods and services on both short and long
October 2002
WHAT DO FRESHWATER ECOSYSTEMS NEED?
1253
FIG. 2. Hydrologic characteristics for the Gunnison River, Colorado, USA (site no. 09128000; USGS Water Resources
Data of the United States [Online, URL: ^http://water.usgs.gov/nwis&]). (a) Daily mean streamflow for the period 1911–2000.
Dashed lines show mean maximum and minimum pre-dam-construction annual flows. (b) Time of year for peak annual
discharge in the Gunnison River, showing April–June snowmelt-driven discharge until dam closure in 1968, when discharge
maxima switched to the period October–March, reflecting water releases for hydroelectric power generation. (c) Daily
hydrograph for pre-dam period 1945–1957. (d) Daily hydrograph for post-dam period 1975–1977. Methods of restoring a
natural-type flow regime call for establishing a range of natural variation for parameters including maximum and minimum
flows, and their timing (Stanford et al. 1996, Richter et al. 1997)
time scales (see Carpenter and Gunderson 2001). Further, introduction of nonnative species that can thrive
under the existing or altered range of environmental
variation can severely modify food-web structure and
processes such as nutrient cycling (e.g., Townsend
1996, van der Zanden and Rasmussen 1996). Exotic
species are often successful in modified systems, where
they can be difficult to eradicate (Moyle and Light
1996).
POSITIVE STEPS TOWARD RESTORATION
Despite the widespread degradation of freshwater
ecosystems, tools are available that can be and are being used to restore aquatic ecosystems to a more natural
and sustainable state, or to prevent continued loss of
biodiversity, ecosystem functions, and ecological integrity. Based on understanding that riverine systems
are naturally dynamic (Poff and Ward 1989, Poff et al.
1997), new statistical approaches toward setting management targets for temporal streamflow variability
have been applied or proposed for several rivers, including the Flathead River (Montana, USA), the Ro-
anoke River (North Carolina, USA) and the Colorado
River (Wyoming, Colorado, Utah, New Mexico, Arizona, Nevada, and California, USA; Sonora, Mexico)
(Richter et al. 1996, 1997, Stanford et al. 1996, Stibrich
and Charles 2000). Variable-streamflow techniques
seek a balance between water delivery needs for power
generation or irrigation, and aquatic ecological needs
for hydrological variability and associated characteristics of timing, frequency, duration, and rate of change
(Fig. 2). They help to reconnect dynamic riparian and
groundwater systems with surface flows, enabling water to move more naturally through the longitudinal,
lateral, and vertical spatial dimensions that are essential
to fully functional ecosystems.
Point sources of water pollution are readily identified, and many have been controlled, due in large part
to the federal Clean Water Act and Safe Drinking Water
Acts. Non-point sources of nutrients and toxins now
supply the majority of pollutants to freshwater ecosystems (CEQ 1995, Carpenter et al. 1998). In some
situations, agricultural runoff has been reduced by using best management practices (BMP), including con-
ESA REPORT
1254
Ecological Applications
Vol. 12, No. 5
FIG. 3. Two different approaches for establishing a regional reference-condition value for freshwaters. Reference-condition
values can be selected from waters that are representative of the most pristine (or least disturbed) condition. If this goal is
unrealistic, or if undisturbed water bodies no longer exist in the region, the reference-condition value can be selected from
among the least disturbed and lowest-nutrient-concentration water bodies found in the region. Surveys of existing water quality
from a broad range of water bodies are necessary in order to establish realistic water quality goals. (Figure modified from EPA
2000: Fig. 6.1.)
trol of erosion and moderate applications of fertilizers,
pesticides, and herbicides. BMPs, however, require
willing farmers whose willingness is often directly proportional to economic incentives, or to the potency of
the regulatory alternative (Young and Karkoski 2000).
Total maximum daily load, or TMDL, is a calculation
of the maximum amount of a pollutant that a waterbody
can receive and still meet water quality standards (EPA
1999). The U.S. Environmental Protection Agency has
recently published guidelines for establishing acceptable nutrient criteria for different ecoregions of the
United States, recognizing the inherent variability in
local and regional availability of nutrients (EPA 2000).
To allow for this natural variation, phosphorus and nitrogen water quality standards are established within
each region based on comparison with reference, or
minimally impacted, waters, or on a percentile of the
lowest-nutrient waters (Fig. 3). Once a standard is set,
management practices can be enacted that reduce inputs
of unwanted nutrients. Atmospheric deposition is an
additional large source of non-point pollution that
could be reduced through more stringent controls on
emissions of sulfur, nitrogen, metals, and organic toxins, and application of more efficient technologies for
transportation and energy production (Driscoll et al.
2001).
CHALLENGES AHEAD
The problems confronting freshwater ecosystems are
intractable if they are approached piecemeal. At least
several government programs have attempted to intervene to prevent drastic ecosystem alteration. But these
programs, such as the EPA Clean Lakes Program
(Clean Water Act: Section 319(H)), the Wetlands Restoration Act of 1998, and even the Endangered Species
Act of 1973, are narrow in their focus, effectively addressing symptoms, rather than the root cause of aquat-
ic-ecosystem decline. Control of pollution is necessary,
for instance, but insufficient for maintaining a native
species assemblage if the water is not available at the
right time, or if invasive species have been allowed to
take hold. The needs of aquatic ecosystems, and the
needs of society for water must be addressed collectively in order for ecological integrity to be maintained
or restored. Politically, this requires broad coalitions
of water users working together toward a mutually acceptable future (Kates et al. 2001).
The best time to develop these coalitions is before
there are water allocation and ecological crises, but for
many parts of the world these opportunities were
missed long ago. There is potential for restoration or
naturalization, however, and examples from Iowa, California, Colorado, the Great Lakes region, and the Pacific Northwest show promise (Rickert et al. 1975,
Rickert 1984, NRC 1992, Rhoads et al. 1999, Bloczynski et al. 2000, Rieman et al. 2000, Young and
Karkoski 2000). An ambitious example is the South
Florida ecosystem, where water control structures are
being physically removed and nutrient inputs curtailed
in an attempt to encourage a more natural system (Box
2).
The ecological consequences that come from depriving freshwater aquatic ecosystems of adequate water quantity, timing, and quality often become apparent
to people only after those consequences begin to interfere with societal uses of freshwater. Nuisance algal
blooms and loss of commercial or sport fisheries are
examples of failures in ecosystem processes that were
often years in the making. Some ecosystems have high
interannual variation in environmental regimes and
ecological states that mask gradual change in physical
and chemical factors. Others are resilient to a certain
amount of disturbance, allowing the persistence of biological relationships within a range of fluctuations in
October 2002
WHAT DO FRESHWATER ECOSYSTEMS NEED?
1255
BOX 2. South Florida Case Study
The south Florida ecosystem covers ;47 000 km2 (18 000 square miles) ranging from Orlando in the
north to the Florida Keys at its southern extreme. It includes the Kissimmee River, Lake Okeechobee,
Everglades National Park, and Florida Bay. The landscape is essentially flat; the elevation drop from Lake
Okeechobee to Florida Bay, a distance of 161 km (100 miles), is ,6.1 m (20 feet). South Florida has
undergone enormous changes in population, land use, and hydrology over the past 100 yr, resulting in
profound changes to ecosystem structure and function. The channelization of the Kissimmee River caused
the loss of 11 000 ha (33 000 acres) of floodplain habitat (Koebel 1995). Accelerated eutrophication of
Lake Okeechobee from runoff associated with dairy and beef cattle operations shifted algal, invertebrate,
and macrophyte composition (Havens et al. 1996, Steinman et al. 1999). Phosphorus enrichment of the
northern Everglades from sugar cane farms has changed periphyton structure and biomass, while increasing
cattail at the expense of sawgrass (McCormick et al. 1996, Newman et al. 1996). Changes in the discharge
of water to estuaries has resulted in massive diebacks of seagrass, because of either too much (Kraemer
et al. 1999) or too little (Sklar and Browder 1998) freshwater.
Efforts began in the early 1900s to drain the Everglades wetlands, which were viewed as wastelands
and useless swamp. Hurricanes and floods prompted massive water management projects. There are now
.2600 km (1600 miles) of levees and canals, 150 gates and other water-control structures, and 16 major
pump stations. The flood control system has worked remarkably well, making the region less vulnerable
to the extremes of flooding and drought by storing water for supply and moving it for flood control.
Management projects were designed in the 1950s when it was anticipated the population in the region
would be two million by the year 2000. Today, the region is home to over six million people. More
significantly, the water projects were not designed with environmental protection or enhancement in mind.
Although it is not possible to restore this region to its pristine condition, efforts are underway to redesign
the south Florida environment to make it more compatible with the way the system used to function.
Environmental problems unintentionally created by water management projects include (1) up to 6.4 3
106 m3 (1.7 billion gallons) per day of excess rainwater channeled directly to the ocean to keep urban and
agricultural lands from flooding, causing salinity imbalances in estuaries and influencing biota; (2) Lake
Okeechobee treated as a reservoir for water supply or flood control, instead of as a natural lake; (3) altered
water supply and periodicity for the Everglades, greatly harming biota; and (4) deteriorated water quality
throughout the region. Approximately 50% of the historic Everglades has been converted to agricultural
or urban use. Populations of wading birds have been reduced 85–90%. Sixty-eight species of plants and
animals in south Florida are threatened or endangered, and invasive species, such as melaleuca, Brazilian
pepper, Australian pine, torpedo grass, Old World climbing fern, and Asian swamp eel are threatening
native habitat and species.
As a result of these environmental problems, the U.S. Congress directed efforts to develop a Comprehensive Everglades Restoration Plan (Water Resources Development Act of 2000), an ambitious, innovative
partnership that includes the goals of enhancing the region’s ecological and economic values, as well as
its social well-being. The objectives of restoration activities are to increase the amount of water available
by storing it instead of sending it out to sea, ensure adequate water quality, and reconnect the parts of
this ecosystem that have been disconnected and fractured. A multi-faceted approach has been proposed
that may take 25 yr or more to implement.
The ecological goals of the plan are to increase the total spatial extent of natural areas, improve habitat
and functional quality, and improve native species richness and biodiversity. Success will be evaluated
with quantitative criteria, such as a goal for Lake Okeechobee of reducing the water column concentration
of total phosphorus from a current concentration of 110 to 40 mg/L. Rigorous programs of scientific
research will continue throughout project implementation, so that major uncertainties can be addressed.
This information, combined with results from the monitoring networks, will be evaluated so that the plan
can be adaptively managed.
ESA REPORT
1256
biotic elements and ecological processes. Once a
threshold is reached, however, freshwater ecosystems
may change rapidly to a new stable condition that is
very difficult to restore back to previous natural conditions (Holling 1973, Sparks 1992, Scheffer et al.
1993). Fishery collapse and permanent cultural eutrophication from nutrient inputs are two examples (Walters 1986, Scheffer et al. 1993, Carpenter et al. 1998).
Monitoring of biological and physical condition, coupled with understanding of ecological dynamics, can
aid detection of problems before they become critical.
TOWARD
A
AND
BALANCE BETWEEN HUMAN USE
NEEDS OF FRESHWATERS
The sustainability of aquatic ecosystems can best be
ensured with a variable flow regime, adequate sediment
and organic matter inputs, natural fluctuations in heat
and light, clean water, and a naturally diverse biotic
community. Failure to provide for these natural requirements results in loss of species and ecosystem
services in wetlands, rivers, and lakes. Aquatic ecosystems can be protected or restored by recognizing
the following:
1) Aquatic ecosystems are connected strongly to terrestrial environments, rather than isolated bodies
or conduits. Further, aquatic ecosystems are connected to each other.
2) Dynamic patterns of flow that are maintained
within the historical range of variation will promote the integrity and sustainability of freshwater
aquatic systems.
3) Aquatic ecosystems additionally require sediments, thermal and light properties, chemical and
nutrient inputs, and biotic populations to fluctuate
within natural ranges, neither experiencing excessive excursions from their historical ranges,
nor being held at constant, and therefore unnatural, levels.
It is one thing to state the requirements for maintaining aquatic-ecosystem integrity. It is another to enact these concepts in the context of today’s complicated
society. U.S. water policy currently supports increased
exploitation of water supplies in order to meet demand,
and maintenance of water quality and flow primarily
as they relate to human health (Gleick 1998). But the
age of ever-increasing exploitation is over. We must
redefine water use based on a finite supply and inclusion of freshwater ecosystem needs (Postel 2000). For
these reasons we offer the following recommendations
for how water is viewed and managed:
1. Incorporate freshwater ecosystem needs, particularly variable flow regimes, into national and regional
water management policies, along with water quality
and quantity.—The dialog that will bring about a
change in water policies can succeed if conducted in
a nonconfrontational manner. It must be based on mu-
Ecological Applications
Vol. 12, No. 5
tual respect of the many users of freshwater. The best
solutions to problems are nearly always proposed by
those directly involved and affected, and since most
land and water use decisions are made locally, we recommend empowering local groups and communities to
implement sustainable water policies. A large and
growing number of watershed groups are already moving in this direction, with support and guidelines from
state and federal agencies (Western Water Policy Review Advisory Commission 1998). Flexibility, innovation, and incentives—such as tax breaks, development permits, conservation easements, and pollution
credits—are effective tools for achieving freshwater
ecosystem sustainability goals.
2. Define water resources to include watersheds, so
that freshwaters are viewed within a landscape, or systems context.—Many of the problems facing freshwater
aquatic ecosystems come from outside the water body.
While this is already recognized by some, agencies and
laws lag behind. One place to initiate a change is
through existing governmental permitting processes.
Renewal requests to the Federal Energy Regulatory
Commission (FERC), permit requests to the Army
Corps of Engineers for dredge and fill operations under
the Clean Water Act Section 404, and land-use and
effluent-discharge permit requests to state, county, and
local entities are ideal occasions to integrate ecosystem
needs alongside the traditional water uses. The Environmental Protection Agency’s TMDL (total maximum
daily load) Program is an effort to address both point
and non-point source pollutants to a water body but
has not yet been fully implemented.
3. Increase communication and education across
disciplines.—Interdisciplinary training and experience—particularly for engineers, hydrologists, economists, and ecologists—will foster a new generation of
water managers and users that think about freshwaters
as systems with ecological purposes as well as watersupply functions.
4. Increase restoration efforts for wetland, lake, and
river ecosystems, using ecological principles as guidelines (NRC 1992).—Some restoration has occurred, but
a greater effort is required to restore the ecological
integrity of the nation’s water resources (EPA 1998).
Restoration is the reinstatement of driving ecological
forces, but many wetland restoration projects, for example, have not gone beyond the mere replanting of
vegetation, ignoring underlying hydrologic and geomorphic, biotic, and biogeochemical processes (Society of Wetland Scientists 2000, Malakoff 1998). Successful restoration projects may even foster complacency among the public. A recent Gallup Poll (22 April
1999) found that Americans are increasingly satisfied
with the nation’s environmental-protection efforts,
making them less likely to support the monetary and
political effort to enact further change (Saad 2000).
The extent of restoration and protection applied to each
October 2002
WHAT DO FRESHWATER ECOSYSTEMS NEED?
water body will be a hotly debated topic, since active
management is inherently a social process (Rhoads et
al. 1999). Restoration efforts encompass a spectrum
from close to full recovery of native species and environmental conditions, to naturalization, whereby
aquatic systems are managed to maintain dynamic, biologically diverse aquatic ecosystems that do not necessarily resemble native ecosystems (Rhoads et al.
1999).
5. Maintain and protect the remaining minimallyimpaired freshwater ecosystems we have.—Aldo Leopold said, ‘‘If the biota, in the course of aeons, has
built something we like but do not understand, then
who but a fool would discard seemingly useless parts?
To keep every cog and wheel is the first precaution of
intelligent tinkering’’ (Leopold 1949:190). Many restoration projects fail to reestablish ecosystem function
once major processes have been disturbed. It is far
wiser and cheaper to conserve what we have. Functionally intact freshwater ecosystems can provide a
source of propagules/colonists for restoration projects
elsewhere.
6. Bring the ecosystem concept home.—Society has
been taught to think about the environment as something somewhere else. Ecological processes are viewed
as occurring in remote and exotic places, not as essential to our daily lives, or strongly influenced by our
actions. Ecosystem sustainability requires that human
society recognize, internalize, and act upon the interdependence of people and the environment of which
they are a part. For freshwaters, this will require broad
recognition of the sources and uses of water for societal
and ecological needs. It will also require taking a much
longer view of water processes. Water delivery systems, and even dams, are developed with lifespans and
management guidelines of decades to at most a century.
Freshwater ecosystems have evolved over much longer
periods of time, and their sustainability must be considered from a long perspective. Governmental policies, mass media, and a market-driven economy all
focus more on perceived short-term benefits. Local watershed groups interested in protecting their natural resources provide a good first step toward long-term
stewardship. They need to be matched by state and
national acknowledgment that fundamental human
needs for water can only be sustained through policies
that preserve the life-support systems of aquatic ecosystems.
Abramovitz (1996:10) described freshwater ecosystems as ‘‘biological assets [that are] both disproportionately rich and disproportionately imperiled.’’ They
need not be so threatened. By inclusion of the need for
naturally varying flow regimes, and reduced pollutant
and nutrient inputs, natural freshwater aquatic ecosystems can be maintained or restored to a sustainable
state that continues to provide the amenities and ser-
1257
vices society has come to expect, as well as helping
native species to flourish.
ACKNOWLEDGMENTS
This paper benefited from discussions with Neil Grigg,
Alan Covich, Rhonda Kranz, Dennis Ojima, and reviews from
Penny Firth, Lou Pitelka, Stuart Findlay, Steve Carpenter,
Pam Matson, Julie Denslow, and the Public Affairs Committee of the Ecological Society of America.
LITERATURE CITED
Abramovitz, J. N. 1996. Sustaining freshwater ecosystems.
Pages 60–77 in L. R. Brown, editor. State of the world
1996. W. W. Norton, New York, New York, USA.
Allan, J. D. 1995. Stream ecology: structure and function of
running waters. Chapman and Hall, London, UK.
Anderson, R. M., B. F. Hobbs, J. F. Koonce, and A. B. Locci.
2001. Using decision analysis to choose phosphorus targets
for Lake Erie. Environmental Management 27:235–252.
Auble, G. T., J. M. Friedman, and M. L. Scott. 1994. Relating
riparian vegetation to present and future streamflows. Ecological Applications 4:544–554.
Auer, M. T., S. W. Effler, and 24 coauthors. 1996. Biology.
Pages 384–534 in S. W. Effler, editor. Limnological and
engineering analysis of a polluted urban lake: prelude to
environmental management of Onondaga Lake, New York.
Springer-Verlag, New York, New York, USA.
Benke, A. C. 1990. A perspective on America’s vanishing
streams. Journal of the North American Benthological Society 9:77–88.
Bloczynski, J. A., W. T. Bogart, B. F. Hobbs, and J. F. Koonce.
2000. Irreversible investment in wetlands preservation: optimal ecosystem restoration under uncertainty. Environmental Management 26:175–193.
Carpenter, S. R., N. F. Caraco, D. L. Correll, R. W. Howarth,
A. N. Sharpley, and V. H. Smith. 1998. Nonpoint pollution
of surface waters with phosphorous and nitrogen. Ecological Applications 8:559–568.
Carpenter, S. R., and L. H. Gunderson. 2001. Coping with
collapse: ecological and social dynamics in ecosystem management. BioScience 51:451–457.
CEQ [Council on Environmental Quality]. 1995. Environmental quality. 1994–1995 Report. Office of the White
House, Washington, D.C., USA.
CEQ [Council on Environmental Quality]. 1996. Along the
American River, the 1996 report of the CEQ. Office of the
White House, Washington, D.C., USA.
Clean Water Act. 1972, and as amended: U.S. Code title 33,
sections 1251–1387.
Cummins, K. W. 1974. Stream ecosystem structure and function. BioScience 24:631–641.
Daily, G. C., editor. 1997. Nature’s services: societal dependence on natural ecosystems. Island Press, Washington,
D.C., USA.
Dobson, A. P., J. P. Rodriguez, W. M. Roberts, and D. S.
Wilcove. 1997. Geographic distribution of endangered species in the United States. Science 275:550–553.
Driscoll, C. T., G. B. Lawrence, A. J. Bulger, T. J. Butler, C.
S. Cronan, C. Eagar, K. F. Lambert, G. E. Likens, J. L.
Stoddard, and K. C. Weathers. 2001. Acidic deposition in
the Northeastern United States: sources and inputs, ecosystem effects, and management strategies. BioScience 51:
180–198.
Endangered Species Act of 1973. 28 December 1973. U.S.
Code title 16, sections 1531–1544.
EPA [U.S. Environmental Protection Agency]. 1998. National water quality inventory: 1996 Report to Congress.
EPA841-R-97-008. U.S. Environmental Protection Agency, Washington, D.C., USA.
EPA [U.S. Environmental Protection Agency]. 1999. EPA
1258
ESA REPORT
Office of Water Total Maximum Daily Load (TMDL) Program. [Online, URL: ,http://www.epa.gov/owow/tmdl/intro.html..]
EPA [U.S. Environmental Protection Agency]. 2000. Nutrient criteria technical guidance manual: lakes and reservoirs.
EPA-822-B00-001. U.S. Environmental Protection Agency, Washington, D.C., USA.
Fisher, S. G. 1983. Succession in streams. Pages 7–27 in J.
R. Barnes and G. W. Minshall, editors. Stream ecology:
application and testing of general ecological theory. Plenum Press, New York, New York, USA.
Gleick, P. H., editor. 1993. Water in crisis: a guide to the
world’s fresh water resources. Oxford University Press,
New York, New York, USA.
Gleick, P. H. 1998. Water in crisis: paths to sustainable water
use. Ecological Applications 8:571–579.
Glenn, E. P., C. Lee, R. Felger, and S. Zengel. 1996. Effects
of water management on the wetlands of the Colorado River
Delta, Mexico. Conservation Biology 10:1175–1186.
Gregory, S. V., F. J. Swanson, W. A. McKee, and K. W.
Cummins. 1991. An ecosystem perspective of riparian
zones. BioScience 41:540–551.
Hairston, N. G., Jr., L. J. Perry, A. J. Bohonak, M. Q. Fellows,
C. M. Kearns, and D. R. Engstrom. 1999. Population biology of a failed invasion: paleolimnology of Daphnia exilis in upstate New York. Limnology and Oceanography
44:477–486.
Havens, K. E., N. G. Aumen, R. T. James, and V. H. Smith.
1996. Rapid ecological changes in a large subtropical lake
undergoing cultural eutrophication. Ambio 25:150–155.
Holling, C. S. 1973. Resilience and stability of ecological
systems. Annual Review of Ecology and Systematics 4:1–
23.
Holling, C. S. 1986. The resilience of terrestrial ecosystems:
local surprise and global change. Pages 292–317 in W. C.
Clark and R. E. Munn, editors. Sustainable development
of the Biosphere. Cambridge University Press, Cambridge,
UK.
Hupp, C. R., and W. R. Osterkamp. 1985. Bottomland vegetation distribution along Passage Creek, Virginia, in relation to fluvial landforms. Ecology 66:670–681.
Hynes, H. B. N. 1970. The ecology of running waters. University of Toronto Press, Toronto, Ontario, Canada.
Jackson, R. B., S. R. Carpenter, C. N. Dahm, D. M. McKnight,
R. J. Naiman, S. L. Postel, and S. W. Running. 2001. Water
in a changing world. Ecological Applications 11:1027–
1045.
Karr, J. R. 1991. Biological integrity: a long-neglected aspect
of water resource management. Ecological Applications 1:
66–85.
Kates, R. W., et al. 2001. Environment and development:
sustainability science. Science 292:641–642.
Koebel, J. W., Jr. 1995. An historical perspective on the Kissimmee River restoration project. Restoration Ecology 3:
149–159.
Kowalewski, M., G. E. A. Serrano, K. W. Flessa, and G. A.
Goodfriend. 2000. Dead delta’s former productivity: two
trillion shells at the mouth of the Colorado River. Geology
28:1059–1062.
Kraemer, G. P., R. H. Chamberlain, P. H. Doering, A. D.
Steinman, and M. D. Hanisak. 1999. Physiological responses of transplants of the freshwater angiosperm Vallisneria americana along a salinity gradient in the Caloosahatchee Estuary (Southwestern Florida). Estuaries 22:
138–148.
Leopold, A. 1949. A Sand County almanac. Oxford Press,
New York, New York, USA.
Likens, G. E. 1984. Beyond the shoreline: a watershed-ecosystem approach. International Verhandlungen für Theo-
Ecological Applications
Vol. 12, No. 5
retische und Angewandte Limnologie, Verhandlungen 22:
1–22.
Malakoff, D. 1998. Restored wetlands flunk real-world test.
Science 280:371–372.
McCormick, P. V., P. S. Rawlik, K. Lurding, E. P. Smith, and
F. H. Sklar. 1996. Periphyton–water quality relationships
along a nutrient gradient in the northern Florida Everglades.
Journal of the North American Benthological Society 15:
433–449.
Meffe, G. K. 1984. Effects of abiotic disturbance on coexistence of predator–prey fish species. Ecology 65:1525–
1534.
Moyle, P. B., and R. A. Leidy. 1992. Loss of biodiversity in
aquatic ecosystems: evidence from fish faunas. Pages 127–
169 in P. L. Fiedler and S. K. Jain, editors. Conservation
biology: the theory and practice of nature conservation,
preservation, and management. Chapman and Hall, New
York, New York, USA.
Moyle, P. B., and T. Light. 1996. Biological invasions of
fresh water: empirical rules and assembly theory. Biological Conservation 78:149–161.
Mulder, C. P. H., D. D. Uliassi, and D. F. Doak. 2001. Physical
stress and diversity–productivity relationships: the role of
positive interactions. Proceedings of the National Academy
of Science (USA) 98:6704–6708.
Naiman, R. J., editor. 1992. Watershed management: balancing sustainability and environmental change. SpringerVerlag, New York, New York, USA.
Naiman, R. J., S. E. Bunn, C. Nilsson, G. E. Petts, G. Pinay,
and L. C. Thompson. In press. Legitimizing fluvial ecosystems as users of water: an overview. Environmental
Management.
Nesler, T. P., R. T. Muth, and A. F. Wasowicz. 1988. Evidence
for baseline flow spikes as spawning cues for Colorado
squawfish in the Yampa River, Colorado. American Fisheries Society Symposium 5:68–79.
Newman, S., J. B. Grace, and J. W. Koebel. 1996. Effects of
nutrients and hydroperiod on Typha, Cladium, and Eleocharis: implications for Everglades restoration. Ecological
Applications 6:774–783.
NRC [National Research Council]. 1992. Restoration of
aquatic ecosystems: science, technology, and public policy.
National Academy Press, Washington, D.C., USA.
Palmer, M. A., A. P. Covich, S. Lake, P. Biro, J. J. Brooks,
J. Cole, C. Dahm, J. Gibert, W. Goedkoop, K. Martens,
and J. Verhoeven. 2000. Linkages between aquatic sediment biota and life above sediments as potential drivers of
biodiversity and ecological processes. BioScience 50:
1062–1075.
Patten, D. T., D. A. Harpman, M. I. Voita, and T. J. Randle.
2001. A managed flood on the Colorado River: background, objectives, design, and implementation. Ecological
Applications 11:635–643.
Pickett, S. T. A., V. T. Parker, and P. L. Fiedler. 1992. The
new paradigm in ecology: implications for conservation
biology above the species level. Pages 66–88 in P. F. Fiedler
and S. K. Sain, editors. Conservation biology. Chapman
and Hall, New York, New York, USA.
Poff, N. L., J. D. Allan, M. B. Bain, J. R. Karr, K. L. Prestegaard, B. D. Richter, R. E. Sparks, and J. C. Stromberg.
1997. The natural flow regime: a paradigm for river conservation and retoration. BioScience 47:769–784.
Poff, N. L., and J. V. Ward. 1989. Implications of streamflow
variability and predictability for lotic community structure:
a regional analysis of streamflow patterns. Canadian Journal of Fisheries and Aquatic Sciences 46:1805–1818.
Postel, S. 1996. Forging a sustainable water strategy. Pages
40–59 in L. R. Brown, et al. State of the world, 1996. W.
W. Norton, New York, New York, USA.
October 2002
WHAT DO FRESHWATER ECOSYSTEMS NEED?
Postel, S. 2000. Entering an era of water scarcity: the challenges ahead. Ecological Applications 10:941–948.
Postel, S. L., G. C. Daily, and P. R. Ehrlich. 1996. Human
appropriation of renewable fresh water. Science 271:785–
788.
Postel, S. L., J. I. Morrison, and P. H. Gleick. 1998. Allocating fresh water to aquatic ecosystems: the case of the
Colorado River Delta. Water International 23:119–125.
Power, M. E., M. S. Parker, and T. J. Wootton. 1996. Disturbance and food chain length in rivers. Pages 286–297
in G. A. Polis and K. O. Winemiller, editors. Food webs:
integration of patterns and dynamics. Chapman and Hall,
New York, New York, USA.
Rhoads, B. L., D. Wilson, M. Urban, and E. E. Herricks.
1999. Interaction between scientsts and nonscientists in
community-based watershed management: emergence of
the concept of stream naturalization. Environmental Management 24:297–308.
Ricciardi, A., and J. B. Rasmussen. 1999. Extinction rates
of North American freshwater fauna. Conservation Biology
13:1220–1222.
Richter, B. D., J. V. Baumgartner, J. Powell, and D. P. Braun.
1996. A method for assessing hydrologic alteration within
ecosystems. Conservation Biology 10:1163–1174.
Richter, B. D., J. V. Baumgartner, R. Wigington, and D. P.
Braun. 1997. How much water does a river need? Freshwater Biology 37:231–249.
Rickert, D. A. 1984. Use of dissolved oxygen modeling results in the management of river quality. Journal of the
Water Pollution Control Federation 56:94–101.
Rickert, D. A., W. G. Hines, and S. W. McKenzie. 1975.
Planning implications of dissolved oxygen depletion in the
Willamette River, Oregon. Pages 70–84 In W. Whipple, Jr.,
editor. Urbanization and water quality control. American
Water Resources Association, Minneapolis, Minnesota,
USA.
Rieman, B. E., D. C. Lee, R. F. Thurow, P. F. Hassburg, and
J. R. Sedell. 2000. Toward an integrated classification of
ecosystems: defining opportunities for managing fish and
forest health. Environmental Management 25:425–444.
Saad, L. 2000. Environmental concern wanes in 1999 Earth
Day poll. Gallup News Service. The Gallup Organization,
Princeton, New Jersey, USA.
Safe Drinking Water Act. 1974, and as amended. U.S. Code
title 42, subsections 300f et seq.
Scheffer, M., S. H. Hosper, M.-L. Meijer, B. Moss, and E.
Jeppesen. 1993. Alternate equilibria in shallow lakes.
Trends in Ecology and Evolution 8:275–279.
Scott, M. L., G. T. Auble, and J. M. Friedman. 1996. Flood
dependency of cottonwood establishment along the Missouri River, Montana, USA. Ecological Applications 7:
677–690.
Scott, M. L., P. B. Shafroth, and G. T. Auble. 1999. Responses
of riparian cottonwoods to alluvial water table declines.
Environmental Management 23:347–358.
Shannon, J. P., D. W. Blinn, T. McKinney, E. P. Benebati, K.
P. Wilson, and C. O’Brian. 2001. Aquatic food base response to the 1996 test flood below Glen Canyon Dam,
Colorado River, Arizona. Ecological Applications 11:672–
685.
Sklar, F. H., and J. A. Browder. 1998. Coastal environmental
impacts brought about by alterations to freshwater flow in
the Gulf of Mexico. Environmental Management 22:547–
562.
Society of Wetland Scientists. 2000. Position paper on the
definition of wetland restoration. [Online, URL: ^http://
www.sws.org/wetlandconcerns/restoration.html&.]
Solley, W. G., R. Pierce, and H. A. Perlman. 1998. Estimated
use of water in the United States, 1995. U. S. Geological
1259
Survey Circular 1200. U.S. Geological Survey, Denver,
Colorado, USA.
Sparks, R. E. 1992. The Illinois River–Floodplain ecosystem.
Pages 412–432 in National Research Council. Restoration
of aquatic ecosystems. National Academy of Sciences
Press, Washington, D.C., USA.
Sparks, R. E. 1995. Need for ecosystem management of large
rivers and their floodplains. BioScience 45:168–182.
Stallard, R. F. 1998. Terrestrial sedimentation and the carbon
cycle: coupling weathering and erosion to carbon burial.
Global Biogeochemical Cycles 12:231–257.
Stanford, J. A., J. V. Ward, W. J. Liss, C. A. Frissell, R. N.
Williams, J. A. Lichatowich, and C. C. Coutant. 1996. A
general protocol for restoration of regulated rivers. Regulated Rivers: Research and Management 12:391–413.
Stein, B. A., and S. R. Flack. 1997. 1997 species report card:
the state of U. S. plants and animals. The Nature Conservancy, Arlington, Virginia, USA.
Steinman, A. D., K. E. Havens, N. G. Aumen, R. T. James,
K.-R. Jin, J. Zhang, and B. Rosen. 1999. Phosphorus in
Lake Okeechobee: sources, sinks, and strategies. Pages
527–544 in K. R. Reddy, G. A. O’Connor, and C. L. Schelske, editors. Phosphorus biogeochemistry of subtropical
ecosystems: Florida as a case example. CRC/Lewis, New
York, New York, USA.
Stibrich, J. S., and T. J. Charles. 2000. Resolution of Endangered Species Act issues in permitting the Plateau Creek
pipeline. Journal of the American Water Resources Association 36:1263–1269.
Swanson, F. J., T. K. Kratz, N. Caine, and R. G. Woodmansee.
1988. Landform effects on ecosystem patterns and processes. BioScience 38:92–98.
Tonn, W. M., J. J. Magnuson, M. Rask, and J. Toivonen. 1990.
Intercontinental comparison of small-lake fish assemblages: the balance between local and regional processes. American Naturalist 136:345–375.
Toth, L. A. 1995. Principles and guidelines for restoration
of river/floodplain ecosystems—Kissimmee River, Florida.
Pages 49–73 in J. Cairns, editor. Rehabilitating damaged
ecosystems, Second edition. Lewis Publishers/CRC Press,
Boca Raton, Florida, USA.
Townsend, C. R. 1996. Invasion biology and ecological impacts of brown trout Salmo trutta in New Zealand. Biological Conservation 78:13–22.
U.S. Geological Survey. 1999. The quality of our nation’s
waters—nutrients and pesticides. U.S. Geological Survey
Circular 1225. USGS Information Services, Denver, Colorado, USA.
Van der Leeden, F., F. L. Troise, and D. K. Todd, editors.
1990. The water encyclopedia. Second edition. Lewis,
Chelsea, Michigan, USA.
Van der Valk, A. G. 1981. Succession in wetlands: a Gleasonian approach. Ecology 62:688–696.
Vander Zanden, M. J., and J. B. Rasmussen. 1996. A trophic
position model of pelagic food webs: impact on contaminant bioaccumulation in lake trout. Ecological Monographs
66:451–477.
Vinson, M. R. 2001. Long-term dynamics of an invertebrate
assemblage downstream from a large dam. Ecological Applications 11:711–730.
Vitousek, P. M., H. A. Mooney, J. Lubchenko, and J. M.
Melillo. 1997. Human domination of Earth’s ecosystems.
Science 277:494–499.
Vollenweider, R. A. 1976. Advances in defining critical loading levels for phosphorus in lake eutrophication. Memoirs
of the Institute of Hydrobiology 33:53–83.
Walker, B. H. 1992. Biological diversity and ecological redundancy. Conservation Biology 6:18–23.
Walters, C. 1986. Adaptive management of renewable resources. MacMillan, New York, New York, USA.
1260
ESA REPORT
Ward, J. V. 1989. The four-dimensional nature of lotic ecosystems. Journal of the North American Benthological Society 8:2–8.
Ward, J. V., and J. A. Stanford. 1979. The ecology of regulated streams. Plenum Press, New York, New York, USA.
Water Resources Development Act of 2000. 11 December
2000. Public Law 106-541, Statutes at Large 114:2645–
2786.
Waters, T. F. 1995. Sediment in streams: sources, biological
effects and control. American Fisheries Society Monograph
7. American Fisheries Society, Bethesda, Maryland, USA.
Western Water Policy Review Advisory Commission. 1998.
Water in the West: the challenge for the next century. Final
Ecological Applications
Vol. 12, No. 5
report to
Congress.
[Online: URL ^http://www.
waterinthewest.org/reading/reading.htm&.]
Wetlands and Wildlife Enhancement Act of 1998. 30 October
1998. Public Law 105-312, Statutes at Large 112:2958.
Wetzel, R. G. 1983. Limnology. Saunders, New York, New
York, USA.
Wilson, M. A., and S. R. Carpenter. 1999. Economic valuation of freshwater ecosystem services in the United States:
1971–1997. Ecological Applications 9:772–783.
Wootton, J. T., M. S. Parker, and M. E. Power. 1996. Effects
of disturbance on river food webs. Science 273:1558–1561.
Young, T. F., and J. Karkoski. 2000. Green evolution: Are
economic incentives the next step in nonpoint source pol
lution control? Water Policy 2:151–173.